1. Field of the Invention
The present invention relates to a detector for detecting the focusing state of a light beam. More particularly, the present invention relates to a detector for detecting the focusing state of a light beam transmitted from a photographed object through a photographing lens optical system or a finder lens optical system and detecting this focusing state a plurality of times for a very short time.
2. Description of the Related Art
A so-called phase difference detecting system is mainly used in a focusing state detector at the present time.
In this phase difference detecting system, a light beam from a photographed object is divided into two light beams in two regions symmetrically arranged with respect to a photographing optical axis. Two images are formed by again focusing the light beams in the respective two regions. The mutual relation in position between these two images is then provided to judge a focusing state of the light beam from the photographed object in an image-forming position. Namely, it is judged whether this light beam from the photographed object is focused or not. Further, it is also judged whether the light beam from the photographed object is focused on a front or rear side of a normal focusing position of a lens system. Further, information of a defocusing amount can be obtained when the light beam from the photographed object is focused on the front or rear side of the normal focusing position.
In such a focusing state detector, the above mutual relation in position between the two images is detected by using a photoelectric converting element such as a charge coupled device (CCD). various kinds of operations of the focusing state detector are performed by only electrically processing an output of the photoelectric converting element. Accordingly, it is possible to detect whether there is a focusing state or not and calculate the defocusing amount at a high speed.
In the general focusing state detector, data of the defocusing amount are detected a plurality of times to improve an accuracy in distance measurement and fulfill an operation predicting function. In this case, a phase difference between images formed by light beams in the two regions symmetrically arranged with respect to the optical axis of the lens system about a light beam from the photographed object is calculated by using a photoelectric converting element such as a charge coupled device (CCD) in each of the plural distance measurements. A focus moving amount of a photographing lens is calculated from data of this phase difference. Accordingly, it takes much time to provide the focus moving amount of the photographing lens.
Namely, a first detecting time of the focusing state multiplied by the number of detections of the focusing state (i.e., the number of distance measurements) is required in total to provide the focus moving amount of the photographing lens.
Further, in the general focusing state detector, the phase difference data at the present time are provided on the basis of image data at the present time obtained at a time point of each of the plural distance measurements. Further, the phase difference data at the preceding time are provided on the basis of the image data at the preceding time. A difference between defocusing amounts at the preceding and present times is detected by comparing the phase difference data at the preceding and present times with each other. When there is no change in defocusing amount, it is judged that the photographed object is at rest.
In contrast to this, when the defocusing amount at the present time is changed from that at the preceding time, it is judged that the photographed object is moving in a direction of the optical axis. Thus, the movement of the photographed object is predicted on the basis of this change in defocusing amount.
However, the movement of the photographed object can be accurately predicted when image signals provided at two time points of distance measurement, i.e., at the present and preceding times are coincident with each other and brightness distributions of the light beams incident to the respective photoelectric converting portions are equal to each other. When these image signals are not sufficiently coincident with each other and these brightness distributions are greatly different from each other, it is impossible to accurately predict the movement of the photographed object since an error in operation of the focusing state detector is caused. Therefore, in a certain case, it is judged that the photographed object is moving although the photographed object is at rest.
Further, when the photographed object is moved in a direction perpendicular to the optical axis of the lens system, the two image data are greatly shifted from each other and are not in conformity with each other. Therefore, a great phase difference is caused even when a distance between the photographed object and the lens system is approximately constant, thereby providing data of an incorrect defocusing amount.
A light-receiving section for providing the image data is constructed by a charge coupled device (CCD) so that a dynamic range of a quantity of light incident to the light-receiving section is narrow. To prevent this dynamic range from being narrowed, a monitor photometric section is disposed in the vicinity of the photoelectric converting portions for detecting image data. A charge storing time is controlled in the light-receiving section for detecting image data on the basis of an output of the monitor photometric section to make constant an average charge storing amount of the image data from the light-receiving section. For example, such a structure is shown in Japanese Patent Application Laying Open (KOKAI) No. 62-192732.
In such a structure, there is no special problem when two image data corresponding to light beams in the two regions symmetrically arranged with respect to the optical axis of the lens system are obtained at only specified time points close to each other. However, there is a problem when image data are obtained at each of a plural of time points far from each other and are compared with each other to predict the movement of the photographed object.
Namely, when the movement of the photographed object is predicted by comparing the data of plural phase differences based on the image data obtained at the plural time points, it is necessary that general photoelectric converting characteristics including the electric characteristics of an amplifier are equal to each other at the plural time points. However, when the focusing state detector has a monitor control means, an apparent gain of the monitor control means is changed at the plural time points. Therefore, the image data are different from each other with respect to the same photographed object so that no image data are in conformity with each other at the plural time points, thereby detecting an incorrect defocusing amount.
Further, in the general focusing state detector, a reference area of data is set in advance on the basis of distance information of the photographing lens when the phase difference is calculated by providing a conformity degree of image outputs of two optical systems. Therefore, a means for detecting a position of the photographing lens is indispensable to the focusing state detector. Accordingly, the focusing state detector has a complicated structure and a mechanical means for detecting a position of the photographing lens must be used in this focusing state detector so that an accuracy in detection of the position of the photographing lens is limited. Therefore, it is difficult to accurately set the reference area.